Configuring SSL Certificates for HTTP Server using OpenSSL

The SSL_CTX object uses a method as a connection method. The methods exist in a generic type (for client and server use), a server only types, and a client only type. the method can be of the TLS_method () : TLS_client_method () : TLS_server_method (): 

SSL_CTX_new():
SSL_CTX_new() creates a new SSL_CTX object as framework to establish TLS/SSL or DTLS enabled connections. An SSL_CTX object is a reference counted. Creating an SSL_CTX object for the first time increments the reference count. Freeing it (using SSL_CTX_free) decrements it. When the reference count drops to zero, any memory or resources allocated to the SSL_CTX object are freed. SSL_CTX_up_ref() increments the reference count for an existing SSL_CTX structure.
int SSL_CTX_set_cipher_list(SSL_CTX *ctx, const char *str):
SSL_CTX_set_cipher_list() sets the list of available ciphers for ctx using the control string str. The format of the string is described in ciphers(1). The list of ciphers is inherited by all SSL objects created from ctx.

int SSL_CTX_load_verify_locations(SSL_CTX *ctx, const char *CAfile, const char *CApath);

SSL_CTX_load_verify_locations() specifies the locations for ctx, at which CA certificates for verification purposes are located. The certificates available via CAfile and CApath are trusted.

int SSL_CTX_use_certificate_file(SSL_CTX *ctx, const char *file, int type);

SSL_CTX_use_certificate_file() loads the first certificate stored in file into ctx. The formatting type of the certificate must be specified from the known types SSL_FILETYPE_PEM, SSL_FILETYPE_ASN1.

int SSL_CTX_use_PrivateKey_file(SSL_CTX *ctx, const char *file, int type);

SSL_CTX_use_PrivateKey_file() adds the first private key found in file to ctx. The formatting type of the certificate must be specified from the known types SSL_FILETYPE_PEM, SSL_FILETYPE_ASN1.

int SSL_CTX_check_private_key(const SSL_CTX *ctx);

SSL_CTX_check_private_key() checks the consistency of a private key with the corresponding certificate loaded into ctx. If more than one key/certificate pair (RSA/DSA) is installed, the last item installed will be checked. If e.g. the last item was an RSA certificate or key, the RSA key/certificate pair will be checked.

Creating a PEM file to initialize SSL:
After downloading SSL certificates we get three files majorly:

1. .pem
2. .key
3. .crt

Three files have to be clubbed to make one pem file.
Initial .pem file looks like:
-----BEGIN CERTIFICATE-----
some random characters
-----END CERTIFICATE-----
Initial .crt file looks like:
-----BEGIN CERTIFICATE-----
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
-----END CERTIFICATE-----
Initial .key file contains the private key and it looks like:
-----BEGIN PRIVATE KEY-----
-----END PRIVATE KEY-----
Final File looks like:
-----BEGIN PRIVATE KEY-----
(Your Private Key: your_domain_name.key)
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
(Your Primary SSL certificate: your_domain_name.crt)
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
(Your Intermediate certificate: DigiCertCA.crt)
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
(Your Root certificate: TrustedRoot.crt)
-----END CERTIFICATE-----

References:
https://www.openssl.org/docs/man1.0.2/man3/SSL_CTX_use_PrivateKey_file.html
https://stackoverflow.com/questions/19029647/ssl-ctx-use-privatekey-file-failed